The present invention generally relates to fuel cells and more particularly to a novel anode electrocatalyst for use in a fuel cell. The present invention particularly relates to a high-output fuel cell of the type that supplies fuel containing ethanol as a principal ingredient directly to an anode and to the electrocatalyst that can efficiently oxidize such a fuel containing ethanol as the principal ingredient and supplied directly to the anode. Further, the present invention relates to an electronic device that uses such a fuel cell.
The fuel cell that uses a fuel of hydrogen gas can provide a high output power density, and a polymer electrolyte fuel cell (PEFC) of the type that uses a hydrogen gas as the fuel is expected as the power supply of high speed vehicles such as an electric vehicle (EV) or distributed type power supply system.
Generally, a PEFC uses a cell unit formed of a junction assembly (membrane-electrode assembly: MEA) of a fuel electrode (anode) and an oxygen electrode (cathode) sandwiching a proton exchange membrane (PEM) therebetween. A PEFC is generally constructed in the form of a stack in which a plurality of such cell units are stacked in plural layers.
In such a cell unit, electromotive force (EMF) is obtained by inducing an oxidation reaction and a reduction reaction at the respective electrodes by supplying hydrogen to the anode as a fuel and oxygen or the air to the cathode as an oxidizer. In order to enhance the electrochemical reaction, a platinum catalyst is used in these electrodes, and protons (hydrogen ions) formed at the anode are transported to the cathode through the PEM.
However, such a fuel cell that uses PEFC cannot be utilized effectively by the public unless infrastructure for hydrogen gas fuel supply is deployed. Further, there are many problems to be solved in the technology of PEFC that uses hydrogen gas as a fuel in relation to the storage and transport of the hydrogen fuel and the technology of handling the hydrogen fuel safely. Because of this, it is now recognized that the use of hydrogen fuel is not suitable for the electric power supply of small electronic apparatuses or portable electronic apparatuses of personal use.
In order to overcome these problems, intensive investigations are being made on the fuel cell of so-called reforming type that produces hydrogen by reforming a hydrocarbon fuel such as a liquefied natural gas (LNG) or a methane gas or a liquid fuel such as methanol.
In the fuel cell of the type that reforms such hydrocarbon fuel, however, it is generally not possible to completely eliminate the formation of carbon monoxide (CO) or other impurities from the hydrogen fuel gas during the reformation process of the raw fuel. Such impurities, particularly CO, cause damaging effect to the function of the fuel cell.
The art of reforming for obtaining hydrogen from a methane gas or methanol is the technology of old and an example of this technology will be explained below for the case of using methanol.
As the reformation method of methanol, steam reformation (STR) method and auto thermal reformation method are well known.
The outline of the STR method is shown in FIG. 1.
Referring to FIG. 1, the vapor reformation process of methanol uses the reaction,CH3OH+H2O→CO2+3H2,and includes a reformation step (Steam Reforming) 101 and a CO reduction (CO Removal) step 102.
Here, the CO reduction process 102 includes an H2/CO ratio adjustment process 102A that uses a water gas shift (WGS) reaction and a CO removal process 102B called a partial oxidation reaction process or preferential oxidation (PROX) process.
With this conventional art, the concentration of CO in the hydrogen gas fuel is reduced to below 100 ppm. Further, it is reported that a CO concentration of 10 ppm or less is possible (Kotoue Y., et al., Sangyo To Denki 547, 19, 1998; Donald S. Cameron, Fuel Cells—Science and Technology 2002, Platinum Metals Rev., 2003, 47, (1), 28-31).
With regard to the impurity concentration level of the hydrogen fuel obtained by the reformation of methanol fuel, empirical and analytical predictions are presented for the hydrogen gas fuel as follows.
TABLE 1H272%CO224%N2 4%CO50~100 ppmO20.5% CH4t.q. (trace quantity)HCOOCH3t.q.HCOOHt.q.CH3OHt.q.
It should be noted that Table 1 shows the impurity concentration level in the hydrogen fuel of dry state for the case all the processes from the STR process 101 to the PROX process 102B are applied to the methanol in the reformation process of FIG. 1.
Thus, it can be seen that CO is contained in the reformed gas fuel with the concentration of 50-100 ppm, while other impurities can be disregard except for nitrogen and oxygen.
It is recognized that CO causes deactivation of platinum catalyst and causes damages on the function of the anode electrocatalyst of the fuel cell, and hence the power generation performance of the fuel cell. This effect is known as catalyst poisoning and can occur even when the concentration is 10 ppm or less.
In order to overcome this problem of catalyst poisoning, various technologies are proposed.
For instance, there is proposed a technology that uses platinum-ruthenium (Pt—Ru) alloy catalyst in place of the platinum catalyst for reducing the catalyst poisoning. It is recognized that this technology can successfully prevent the adsorption of CO molecules, which may be contained in the hydrogen gas with several ppm to several ten ppm as impurities and act as a catalyst poison to the platinum surface, thereby reducing the deactivation of the platinum catalyst action.
Aiming for further reduction of the catalyst poisoning in the fuel cell electrode reaction (referred to hereinafter as “reformed gas fuel”), following technologies are proposed.
Japanese Laid-Open Patent Application 10-228912 describes an improved catalyst that uses an assembly of a metal oxide, platinum and a transition element disposed close with each other, or an assembly of a metal oxide, platinum and an element of Group IIIa or IVa, with regard to a reformed gas fuel (Reformate).
Furthermore, Japanese Laid-Open Patent Publication 8-501653 refers to U.S. Pat. Nos. 3,297,484 and 3,369,886 in relation to general construction of catalyst, wherein these references disclose the technological information of various catalyst compositions of metals, metal oxides, metal alloys, metal compounds in relation to the application of new ion exchange membrane to the technology of fuel cell.
Furthermore, Japanese Patent 3,351,285 proposes a catalyst mixture formed of a platinum catalyst, a platinum-ruthenium multicomponent catalyst and a platinum-molybdenum multicomponent catalyst carried separately on a single carbon carrier for use as an anode electrocatalyst capable of reducing the catalyst poisoning caused by CO in a reformed gas fuel.
Japanese Laid-Open Patent Application 10-302810 proposes a catalyst that contains platinum or a platinum alloy and a Lewis acid (solid acid) containing a rare earth metal element as the anode electrocatalyst capable of reducing the poisoning caused by CO in a reformed gas fuel. In this reference, a multicomponent alloy of platinum, a platinum group element and also other metal element is used for the foregoing platinum alloy.
Japanese Laid-Open Patent Application 2001-15121 proposes a ternary alloy catalyst of the Pt—Ru—Mo system and the Pt—Ru—W system as the anode electrocatalyst capable of reducing the poisoning caused by CO in a reformed gas fuel. According to this reference, the reduction of Ru content is achieved while maintaining the effect of poisoning reduction achieved by the Pt—Ru alloy catalyst.
Japanese Laid-Open Patent Application 2001-15122 proposes a binary alloy catalyst of the Pt—W system for the anode electrocatalyst capable of reducing the poisoning caused by CO in a reformed gas fuel. According to this reference, it becomes possible to achieve the poisoning reduction effect similar to the one obtained by the Pt—Ru alloy catalyst without using expensive Ru.
As noted above, it has been a major subject in the conventional fuel cell technology to reduce the poisoning of platinum catalyst caused by CO, while this problem becomes a particularly serious problem in the case of using a reformed gas fuel in place of a hydrogen gas.
Generally, poisoning of a platinum catalyst is understood as the phenomenon in which electrons are back-donated from a platinum electrode to CO molecules and causing a strong adsorption of the CO molecules to the platinum electrode surface.
There is recognized existence of a strong dependence between a hydrogen oxidation activity current and CO coverage rate for a platinum electrode, and from this, it is understood that the phenomenon of CO poisoning of platinum catalyst is actually a surface reaction taking place between the CO molecules contained in the gas phase and the platinum surface. Thereupon, the conventional art of reducing the CO poisoning attempts to maintain the hydrogen oxidation activity of the platinum catalyst surface that has absorbed CO, by oxidizing the CO molecules by way of supplying OH-species thereto. In order to do so, the conventional technology provides a layer of atoms having strong oxygen affinity in the vicinity of the Pt—CO system formed in by the Pt catalyst surface by absorbing CO.
However, there still remain various problems not understood, such as migration of the CO molecules on the adsorbed surface, change of the electron state of the platinum surface covered with CO, and associated change of the coverage rate of CO, even in the case of such a surface reaction taking place in the catalyst metal-gas phase system. Thus, conventional technology cannot provide a quantitative solution to these various phenomena, and there exists no universal technology capable of eliminating the problem of poisoning of platinum catalyst.
In view of the situation noted above, there is emerging the need for a new technology of fuel cell different from the fuel cell of reformation fuel type, particularly in relation to miniaturization of the fuel cell.
In relation to this, a new-type fuel cell called direct methanol fuel cell (DMFC) is drawing attention, wherein DMFC is a fuel cell that conducts power generation with a construction similar to that of a PEFC, except that methanol is supplied directly to the anode for anode oxidation reaction.
A direct methanol fuel cell (DMFC), which is a fuel cell that supplies methanol directly to the anode, is a power generation device operated at a relatively low temperature (normal temperature to 120° C.).
In this fuel cell, which is studied for long time, the methanol fuel is supplied directly to the anode, and thus, the reformation device for extracting hydrogen from alcohol is no longer necessary. Thus, the device can be formed compact and low cost. Further, the overall operation means can be simplified.
Although there are various advantages in a direct methanol fuel cell, there exists also a problem in that the anode oxidation reaction of methanol provides a large overvoltage. In the oxygen-hydrogen fuel cell that uses hydrogen for the fuel, the oxidation rate of hydrogen is large and the oxygen reduction reaction in the cathode becomes the bottleneck. In the case of a direct methanol fuel cell, on the other hand, methanol oxidation becomes a major bottleneck, and the problem of decrease of output is not avoidable. Further, toxicity of methanol can not only harm the human health but also threaten the human life.
In spite of these apparent disadvantages, there exists great expectation in a liquid fuel cell in view of its safety and in view of the easiness of miniaturization of the fuel cell, as compared with the case of using a hydrogen gas fuel.
From the foregoing viewpoint, there have been proposed various fuel cells that use ethanol, dimethylether, propane, and the like, in place of methanol. However, such fuel cells cannot provide the output performance superior to the one obtained in the case methanol is used. Nevertheless, it is recognized that the use of ethanol, which provides little toxicity to human body and similarly little influence to the environment, is evidently most preferable for the fuel for use in a compact fuel cell for personal use.
It is known that a platinum electrode is suitable for electrode oxidation of ethanol. Further, it is also known that an alloy of Pt and Ru performs better (Souza, et al., J. Electroanalytical Chem. 420, pp. 17-20, 1997). Nevertheless, no satisfactory ethanol oxidation has been achieved even in the case the alloy of Pt and Ru is used for the electrode. Because of this, the development of highly efficient electrocatalyst suitable for use in the anode of the direct ethanol fuel cell and simultaneously capable of providing the output characteristic of the direct methanol fuel cell has been eagerly waited for.
The electrode reaction of a fuel cell includes an anode reaction and a cathode reaction.
The cathode reaction is usually an oxygen reduction reaction. Because the cathode reaction is unavoidable, it is required that the anode reaction is a faster reaction (higher rate reaction) than the cathode reaction. In other words it is desired that the anode reaction does not become the overall bottleneck of the electrochemical reaction taking place in the fuel cell.
In the case of using a hydrogen fuel, this demand is satisfied. Nevertheless, the handling of hydrogen fuel cell is complex as noted before, and a hydrogen fuel cell is inappropriate for the power supply of personal usage.
Liquid fuel is a promising candidate replacing hydrogen, and methanol is thought as the leading candidate the liquid fuel.
However, methanol has a problem of large overvoltage at the time of the anode oxidation in any of the cases when it is used alone or when it is used in the form mixed with water. Further, the current that can be taken out from the fuel cell does not reach the value obtained in the case of using hydrogen. In addition to this, ethanol or dimethylether is thought as promising candidates, but these fuels cannot provide the anode oxidation characteristic that exceeds the case in which methanol is used. This can be evidently seen when FIG. 2 in p. 258 of Lamy, et. al. J. Power Sources, 105, 2002, pp. 283-296, is compared with FIG. 8 in p. 806 of Lamy, et al., J. Appl. Electrochemistry, 31, pp. 799-809, 2001.
When the anode oxidation characteristic of methanol and the anode oxidation characteristic of ethanol are compared for the case of the fuel cell having a platinum catalyst, it is noted that there exists a remarkable difference with regard to the magnitude of anode current density, which is remarkably large as compared with the difference of the activation overvoltage. It is believed that this reflects the existence of complicated chemical reactions in the anode oxidation reaction in both of the cases of using methanol and ethanol for the fuel and the function of catalyst cannot be explained by a simple mechanism of CO poisoning as in case of the using a hydrogen gas for the fuel.
Thus, the catalyst poisoning occurring in a DMFC would be very much different from the CO poisoning caused in the case of using a reformation fuel gas.
Thus, the anode oxidation reaction caused in the case of using a fuel of alcohol or alcohol solution has a complicated aspect and cannot be explained by a simple surface reaction taking place between the catalyst metal and gas phase.
With regard to this issue, Zhu et al., Langmuir 2001, 17, pp. 143-154 and Wasmas, et al., J. Electroanalytical Chemistry 461, pp. 14-31, 1999 provide a representative theory. However, there is no established theory accepted by those persons skilled in the art.
Although there is proposed a model of molecule adsorption/desorption taking place at a platinum catalyst surface in FIG. 3, p. 22 of Wasmas, et al., op. cit., in relation to the direct anode oxidation reaction of the methanol, the understanding of adsorption species or the scheme of reaction varies variously among those skilled in the art.
What is common to these conventional references (Lamy, et al., op. cit., Lamy et al., op cit., Zhu, et al., op cit., Wasmas, et al., op cit.) is that there would exist the following electrochemical reactions in the anodic oxidation process of methanol, judging from the identification of adsorbed species and from the results of molecular spectroscopy of decoupled species.
In the case of the catalyst poisoning caused by CO contained with small amount in a reformed gas fuel, the species involved in the catalyst surface reaction are limited to H2, the adsorbed CO molecules, and the catalyst itself, and the problem of the catalyst poisoning is described by the physical chemistry of the system including these species.
In the case of the direct anode oxidation of alcohol, however, the reaction becomes a liquid phase reaction taking place in a solution, irrespective of whether water is involved or not, and various molecular species can become the obstacle of the anode oxidation catalysis, in addition to CO. These species include the alcohol molecules, fragments of the alcohol molecules, and intermediate species and byproducts that are formed by the adsorption/desorption and the electrochemical reaction.
While there are several patent references noted before that assert that the Pt—Ru binary catalyst is effective for eliminating the poisoning, Gasteiger, et al., Electrochemica Acta, 39, 1994, pp. 1825 reports about the optimum content of Ru in this alloy catalyst as follows.
TABLE 2Optimum Ru content in Pt—Ru binary alloy catalyst50 atm %CO oxidation10 atm %Methanol oxidation
As can be seen in Table 2, the optimum Ru content in the Pt—Ru binary alloy catalyst is very much different between the case of the methanol oxidation and the case of the CO oxidation, and this strongly suggests that that the mechanism of the methanol oxidation is different from mechanism of the CO oxidation.
It has been understood that the oxidation of methanol becomes the bottleneck because the carbon monoxide formed during the process of electrode oxidation of methanol is adsorbed on the electrode and poisoning is caused in the electrocatalyst including platinum as a result. In order to solve this, therefore, various electrocatalysts have been studied. It is known that the problem of CO poisoning can reduced by using a Pt—Ru alloy. Even in this case, however, the reaction at the anode is still a bottleneck as compared with the oxygen reduction reaction taking place at the cathode.
In the representative technology shown in Japanese Laid-Open Patent Application 11-510311, which aims for the direct methanol fuel cell, it is noted that the technology merely follows the conventional technology developed for controlling the poisoning caused by CO in a reformation gas by using a binary catalyst.
With regard to the anode oxidation reaction of methanol, there exists no established theory about details of the reaction, even with reference to the foregoing references Lamy, et al., op cit., Lamy et al., op cit, Zhu, et al., op cit., and Wasmas, et al., op. cit.
Thus, one possible reaction process will be represented below for the case of using a Pt—Ru alloy catalyst.
At the anode, there occurs the reaction:CH3OH+H2O→6H++6e−+CO2.
In this anode reaction, the processes below are involved:CH3OH+xPt→PtxCH2OH+H++e−CH2OH+xPt→PtxCHOH+H++e−CHOH+xPt→PtxCOH+H++e−CHOH+xPt→PtxCO+2H++2e−PtxCHOH+PtOH→HCOOH+H++e−+xPt
In this reaction, it should be noted that COH (or HCO) or CO becomes a strong adsorption specie to platinum, while Ru provides the following effect to this adsorbed specie:Ru+H2O→RuOH+H++e−PtxCOH+RuOH→CO2+2H++2e−+xPt+RuPtxCO+RuOH→CO2+H++e−+xPt+Ru.
At the cathode, there takes place the reaction:O2+4H++4e−→2H2O
Thus, even in the case of using methanol, in which only one carbon atom is contained, the anode reaction involves such complicated processes noted above. As ethanol includes two carbon atoms in the structure, the complexity of the anode oxidation reaction process would be incomparable with the foregoing anode oxidation reaction process for methanol.
For example, the description in Lamy et al., J. Appl. Electrochem. 31, pp. 799-809, 2001, op. cit., is easily understood by those skilled in the art.
The anode oxidation reaction of ethanol is represented as:C2H5OH+3H2O→12H++12e−+2CO2.
These active species such as —COH, CO, —COOH and —CH3, which are absorbed on the platinum surface cause the problem of poisoning, are formed from various dissociation species or intermediate species, such as CH3—CH2OH, CH3CHO, CH3COOH, and the like, formed during the anode electrochemical reaction of ethanol.
Thus, the anode oxidation reaction of ethanol is a complex reaction in which various elemental reactions and intermediate reactions are entangled. Because of this, the anode oxidation current of ethanol could not exceed the anode oxidation current of methanol as can be seen in Lamy, et al., op. cit. and Lamy, et al., op cit. Further, as can be seen in Fujita, et al., the Battery Debate Lecture, Abstract 42nd, pp. 590-591, 2001, there has been a situation that only one half or less of the power generation characteristic (output voltage, output density, and the like) of the direct methanol fuel cell is achieved for a single cell of the direct ethanol fuel cell, as long as a conventional Pt—Ru binary catalyst is used. Thus, the technology of direct ethanol fuel cell has not reached the level of practical use yet, apart from its theoretical possibility.
In spite of its lower electrode oxidation rate as compared with a methanol fuel cell, there exists enormous expectation on a compact portable fuel cell for personal use that is most suitably realized by using ethanol for the fuel, in view of various aspects of safety and easiness of sales to the public. However, there has been no known electrocatalyst that can reduce the overvoltage of the reaction and simultaneously increase the current density while using ethanol for the fuel. Thus, there has been a keen demand for such an electrocatalyst suitable for use in a direct ethanol fuel cell.